Technical Field
The present invention relates to a solder alloy. More specifically, the present invention relates to an inexpensive lead-free high-temperature solder alloy used instead of conductive adhesives and in die bonding for bonding of a back side of a device such as a power device.
Background Art
Sn—Pb eutectic solders or Pb-based solder alloys containing 90% by mass or more of lead (Pb) that have been conventionally used contain lead, which is a toxic substance, and therefore, the use of such solders or solder alloys has been restricted. In recent years, Sn—Ag eutectic solders or Sn—Ag—Cu-based solders that do not contain lead have become widespread and have been used for bonding between electronic components and printed circuit boards. However, if a lead-free solder mainly consisting of Sn is used, the soldered part is subjected to a high heat of a temperature as high as 260° C., for example, and therefore, a problem of poor heat resistance, such as melting of electrodes or breaking of wires, may arise with respect to internal bonding of electronic parts.
In addition, in the field of power devices, it has been more and more desired to use power devices at high temperatures, and the specifications of operation temperature required for such power device products have been raised from a conventional operation temperature specification of around 150° C., which is of a level of a low temperature reached by self heating, to 175° C., and then to 200° C. Accordingly, it has been also desired to improve the heat resistance of bonding portions of power devices. In 2011 Report on Results of Environment-Conscious Advanced Packaging Technology (July 2011) by the Japan Electronics and Information Technology industries Association (JEITA), it has been reported that the heat resistance can be secured by conventional techniques by employing a Pb-based composition (a material mainly consisting of lead and having a melting point of 290° C. or higher, for example). According to some other reports, a temperature of 260° C. or higher is required as the heat resistance temperature for portions to be bonded by die bonding, which are used for internal bonding of electronic parts. Sn—Ag—Cu-based solders have become widespread for use as conductive adhesives and Pb-free solders, and with respect to such Sn—Ag—Cu-based solders, the solidus temperature is around 220° C., and therefore Sn—Ag—Cu-based solders may be incited at the required heat resistance temperature of 260° C. described above. Therefore, the problem of poor heat resistance described above, such as melting of electrodes or breaking of wires, may arise in some cases.
Pb-based solders with a high Pb content have a high solidus temperature of 290° C. or higher, which satisfy the required heat resistance; however, the use of lead has been restricted. High-temperature solders, which have a high solidus temperature similarly to the Pb-based solders described above, include solders constituted by a noble metal such as Au-20% Sn (solidus temperature: 280° C.), Au-3.6% Si (solidus temperature: 370° C.), and Au-25% In (solidus temperature: 370° C.). However, because these solder alloys are very expensive, it is difficult to use them commonly as a substitute material for Pb-based solders.
Bi-based alloys include a high-temperature solder produced on the basis of Bi—Ag-based materials (solidus temperature: 262° C.) of which the characteristics have been improved by adding additional elements (see Patent Literatures 1 and 2). However, because the melting point has no sufficient margin from the required heat resistance temperature of 260° C., a problem may arise in that any small excess of the peak temperature over the required heat resistance temperature would cause the material to melt, which may lead to poor bonding.
In addition, a composition is known in which a very small amount of element having a remarkably low eutectic point, such as tin (Sn) or indium (In), is added to bismuth (Bi) (eutectic point of a Bi—Sn eutectic crystal: 139° C., eutectic point of a Bi—In eutectic crystal: 109.5° C.) (see Patent Literature 3). However, if the material is contaminated with Sn or In and even if the amount of Sn or In is as very small as 1,000 ppm or lower, the material may be segregated, and the material may be melted in low melting point phases that may be formed in the segregated portion, which may thereby cause degradation of mechanical characteristics and long-term environmental resistance: i.e., the life of the material may be shortened.
A through-type ceramic condenser has been known in which a solder mainly consisting of Bi and including components such as silver (Ag) or antimony (Sb) that has been loaded thereto is charged into a hole of the structure (see Patent Literature 4). However, such an invention is intended for insertion mounting type components. In addition, the characteristic required for solders is to not volumetrically shrink due to solidification, which is different from the characteristic required for solders for die bonding.
In addition, Pb-free solders for use at high temperatures, which mainly consist of Bi, have been known (see Patent Literature 5). However, such an invention contains zinc (Zn) and Sn as essential constituents, and it is disclosed therein that Bi—Ge-based solders, which do not include Zn or Sn, are not appropriate as solders due to their poor workability and wettability.